1. Field of the Invention
This invention relates to the manufacture of articles of expandable polystyrene and the like, and in particular, it relates to a change in the polymerization recipe by which there are formed beads of expandable vinyl aromatic monomer, e.g., polystyrene, which contains on the order of 5 to 9% of pentane or other blowing agent, and still more particularly, the invention relates to a process for making such beads in which the beads have incorporated in them about 500 to 5000 parts of a Fischer-Tropsch wax of a congealing point between 86.degree. and 110.degree. C.
In other aspects, the invention relates to expandable or expanded thermoplastic-composition products having considerable advantages in respect to various matters, as hereinbelow more fully explained.
2. Description of the Prior Art
Those skilled in the art are familiar with two main practices by which styrene may be polymerized, in a suspension-polymerization process, thereby obtaining beads of polystyrene having a molecular weight on the order of 150,000 to 300,000 and containing on the order of 4 to 8% of pentane as a blowing agent. In one of the known methods, styrene, water, a small proportion of protective colloid, such as polyvinylpyrrolidone, and small but effective proportion of one or more free-radical initiators, such as various peroxides and perbenzoates or the like, singly or in mixtures thereof, are stirred and heated to cause polymerization to take place for beads of desired size. According to another suspension-polymerization procedure, water and styrene are charged to a polymerization kettle, along with an agent such as calcium phosphate, and heated with agitation, an emulsifier being added at a particular stage of the heating to influence the bead size. In either case, pentane or the like is added before, during, or after the polymerization.
The use of additives of various kinds has been proposed, such as the addition of hexabromocyclododecane to inhibit the flammability of the product and to influence its cell structure.
In U.S. Pat. No. 3,647,723, it is taught that styrene should be polymerized in the presence of a wax having a melting point of 70.degree. to 123.degree. C., an acid number of 0 to 45, and a saponification number of from 3 to 150. This is said to reduce the tendency of the beads to stick or clump in the pre-expansion stage in hot water. The process is said to be applicable to suspension polymerization of the kind conducted in the presence of salt, and the patent goes on to teach a further decreasing of the stickiness of the beads by treating the beads with zinc, calcium, or aluminium salt of a fatty acid, such as zinc stearate.
In U.S. Pat. No. 3,320,188, the addition of an ester wax of high melting point, of at least about 10.degree. C. higher than that of the solidification point of the polymer, in an extrusion process of polystyrene or the like, to provide a nucleation effect, is taught.
U.S. Pat. No. 2,979,476 teaches mixing polystyrene with about 1% of microcrystalline or Fischer-Tropsch waxes, to form bulk-polymerized materials which are useful for the manufacture of phonograph records.
U.S. Pat. No. 3,060,138 is limited to making foamable polystyrene particles with the use of isopentane as a blowing agent and the addition of 0.5 to 3% of a paraffinic hydrocarbon having 16 to 46 carbon atoms. According to this patent, the use of isopentane is essential, if a desirably fine-celled product is to be obtained.
U.S. Pat. No. 3,224,984 teaches the addition of 100 to 5,000 parts per million of a polyolefin wax "or similar organic resinous polymeric substance". It teaches that the addition of such material gives a desirably small cell size, such as 80 microns, or about 12 cells per millimeter, implying that this result, which is desirable because it shortens the length of time that it is necessary to keep the articles of expanded polystyrene in the mold and thus in the final molding operation, is brought about by the use of such organic resinous polymeric substance. The preferred material used is a polyolefin, is unlike the Fischer-Tropsch wax used with the present invention, and comprises a mixture of homologous but different molecular species with various C.sub.1 to C.sub.4 side chains. The patent contains no teaching to the effect that a stable cell structure in expandable polystyrene can be obtained, independent from the internal water content of the beads, the polymerization conditions or recipe, or drying and storage conditions of the expandable polystyrene.
It is well known in the industry that the cell structure of beads, when expanded shortly after the polymerization is completed, may vary sharply from the one obtained after flash drying or after storage at different temperatures and/or humidities. Such variations are further enhanced by using different modifiers, like bromine compounds which influence the flammability properties and may lead to a very heterogeneous, non-reproducible cell structure, even after applying the usual techniques of conditioning the product, i.e., subjecting the beads to the action of dry air. A reproducible, uniform cell structure, however, is a prime prerequisite for obtaining products with consistent properties in processing and application.
The subject of the invention is the production of expandable polystyrene with uniform, reproducible cell structure independent from the polymerization recipe, temperature and processing conditions prior to its use in expansion and molding plus the combination with improved expandability, reduction of clumping, improved wet and dry bead flow, improved fusion, reduced water adsorption, increased hydrophobicity, good mold release and excellent smooth surface, in particular of hot-wire-cut boards made from blocks of expandable polystyrene. The combination of such properties has hitherto not been achieved nor taught in the above cited patents.
Thus, in general, it is fair to state that the prior art of which the applicant is aware does not teach or suggest overcoming the various problems indicated above, by the addition of the indicated quantities of Fischer-Tropsch wax.